1. Field of the Invention
This invention relates to high density animal housing systems, and in particular to housing of laboratory animals for isolation from pathogens, in cage units removably mounted to ventilating shelf racks.
2. Description of Prior Art
In high density animal housing installations, cage occupants must be protected from air-borne dust and dander, which are the vectors for various viruses. To achieve high density the cages are often set on shelves one above another, with the drawback that the dust and dander from cages at the upper shelves naturally falls to the lower shelves, especially as the cages are moved around. Such falling material as well as air-borne dust and dander can rapidly spread disease through a colony of laboratory animals such as mice. Infant mice are especially susceptible to fatalities caused by diarrheal infections spread in this manner.
U.S. Pat. No. 3,343,520--Shwarz, Jr., among other, discloses a filter cover in the form of an inverted box of rigidified filter material, to be disposed over the opened top of an otherwise impervious box-like plastic cage rested on a shelf. Such filter tops may be formed of a compressed fibrous material, bonded together and/or having a resin mixed in for rigidity. Published British patent application No. 2,065,440--Bernardini, discloses a filter cover in which a filter is formed of a flexible sheet and affixed to cover apertures in a rigid cover structure. The cover structure holds the material in a shape of an inverted box, air passing through the top and sides of the inverted box or "filter bonnet".
It is essential in filter covers for free-standing cages to provide sufficient surface area that air flow remains adequate to supply oxygen and to remove harmful vapors such as carbon dioxide and ammonia, which build up in the cage and bedding due to the respiration and urine secretion of the housed animals. More than sufficient air flow capacity is preferable to avoid the need for too-frequent changes as the filter cover material becomes clogged with dust and dander. If ammonia and carbon dioxide are allowed to build up, then the animals will be weakened and therefore made susceptible to infection on that account. On the other hand, too porous a filter might admit pathogens carried on dust or dander.
Attempts have been made to localize the filter material to certain areas of a box-like filter cover, for example, as shown in European published application No. 36,628 dated Sept. 30, 1981, and U.S. Pat. No. 4,480,587--Sedlacek. Both teach inverted box covers in which only the bottom of the inverted box includes filter material. These devices are characterized by limited filter surface area and necessarily require a relatively open mesh of filter material or very frequent changing to offset the relatively smaller surface area that is open to air flow. With a high performance close-fiber filter, particularly with time, ammonia and carbon dioxide build up in the cages.
Many attempts have been made to develop laminar flow ventilation of cages to preclude the need for filters. The idea is to develop a moving mass of air in which all portions move in parallel over the cages. Laminar flow has not proved practical because any obstruction disturbs the flow, causing eddies and spreading dust and dander.
In cage systems with downwardly-directed box-like filter covers or bonnets, eddy currents in the cage and in the area of the cage are the driving force for causing passage of air to and from the cage enclosure. These eddy currents are substantially caused by thermal currents from the body heat of cage occupants. In U.S. Pat. Nos. 4,343,261--Thomas; 4,085,705--Gland et al.; and 4,249,482--Harr; cage systems are disclosed in which the cage boxes are mounted under the shelves rather than on top of the shelves, the open tops of the cages being directed against the underside of the shelves. The shelves are provided with integral ducts for at least supply or exhaust of air, a flow being driven to or from the room around the cage/shelf interface. Preferably ducts are included for balanced driving of both supply and exhaust. The shelves are internally lengthwise divided into supply and exhaust ducts, holes to the ducts in the bottom walls of the shelves being encompassed by the open-topped cages, whereby powered air supply and/or exhaust means blow clean fresh air into the cages and exhaust carbon dioxide and fumes.
When open-topped cages are removed from the rack, the occupants are exposed. The aforesaid patent to Thomas teaches a cage cover with integral valves. The cover may be sealably placed over the opened top of the cage and the valves opened by contact with the shelf. When the cage is withdrawn from the rack, upwardly-biased movably bodies in the valves are freed and the valves close automatically. The cage is then sealed airtight. The occupants of the cage are isolated, but the cage is not ventilated. A filter pad may be mounted in each valve, the filter pad being operative only when the valve is open due to pressure against the shelf.
The present invention seeks to achieve the isolation benefits of a covered cage system and also the full ventilation benefits of a powered individually-ventilated cage rack. A filter cover is disclosed that is sealably affixed to the cage and is flat enough to fit between the lower face of a shelf and the opened top of a cage suspended below the shelf. The filter is mounted to a resilient frame adapted to sealably rest around the free standing edges of the open wall of the cage. The filter panel functions as an air seal with the shelf as well as a filter cover for the cage. The filter cover prevents passage of dust and dander between the rack and the cage. When the cages are removed from the rack, they remain safe from air-borne contagion and are at least partly ventilated in a similar manner to inverted box filter bonnets. The system further comprises a service and isolation enclosure producing a downward flow of clean, filtered air over a service area, whereby cages removed from the rack can be opened in the safety of the servicing unit, for changing cage bedding, food, water or occupants. Furthermore, the powered nature of the air flow in both the cage rack and in the service box provide adequate ventilation notwithstanding the relatively smaller surface area of the filter covers across only the opened tops of the cages, as opposed to the larger five wall structure of prior art inverted-box filter bonnets. When a cage is removed from the rack even after use, the filter panel is for the most part clean, any accumulations being restricted to the area of the filter panels aligned with the air holes under the shelves. In this manner, laboratory animals can be isolated from contagion and adequately ventilated at all stages of processing.